<?
$cat_id = getValue('cat_id', 'int', 'POST', 0);
$db_cat_child = new db_query('SELECT cat_id,cat_name FROM categories_multi WHERE cat_active = 1 AND cat_parent_id = ' . $cat_id);
while ($row = mysql_fetch_assoc($db_cat_child->result)) {
    ?>
    <option value="<?= $row['cat_id'] ?>"><?= $row['cat_name'] ?></option>
<?
}